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Re: [Qemu-devel] Number of usable slots in PCIe Root Port / PCIe Switch
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] Number of usable slots in PCIe Root Port / PCIe Switch Downstream Port |
Date: |
Wed, 23 Aug 2017 19:19:14 -0300 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
On Wed, Aug 23, 2017 at 11:16:27PM +0300, Marcel Apfelbaum wrote:
> On 23/08/2017 19:46, Andrea Bolognani wrote:
> > AFAIK some PCI controllers, particularly:
> >
> > * PCIe Root Port (pcie-root-port, ioh3420)
> > * PCIe Switch Downstream Port (xio3130-downstream)
> >
> > only have a single usable slot. libvirt knows about this
> > fact, and will prevent you from adding more than one
> > device to the respective bus.
> >
> > However, as Lukáš recently noticed, QEMU won't complain
> > if you add more devices:
> >
> > $ qemu-system-x86_64 \
> > -nodefaults -nographic \
> > -M q35 -monitor stdio \
> > -device pcie-root-port,id=pci.1 \
> > -device virtio-scsi-pci,bus=pci.1 \
> > -device virtio-scsi-pci,bus=pci.1
> > QEMU 2.9.0 monitor - type 'help' for more information
> > (qemu) info qtree
> > bus: main-system-bus
> > type System
> > [...]
> > dev: q35-pcihost, id ""
> > [...]
> > bus: pcie.0
> > type PCIE
> > dev: pcie-root-port, id "pci.1"
> > [...]
> > bus: pci.1
> > type PCIE
> > dev: virtio-scsi-pci, id ""
> > [...]
> > addr = 01.0
> > bus: virtio-bus
> > type virtio-pci-bus
> > dev: virtio-scsi-device, id ""
> > [...]
> > bus: scsi.1
> > type SCSI
> > dev: virtio-scsi-pci, id ""
> > [...]
> > addr = 00.0
> > bus: virtio-bus
> > type virtio-pci-bus
> > dev: virtio-scsi-device, id ""
> > [...]
> > bus: scsi.0
> > type SCSI
> > (qemu)
> >
>
> Hi Andrea,
>
> > As you can see, all devices will show up in the qtree;
> > only the one with addr=00.0, however, will actually be
> > visible to the guest OS according to my tests.
> >
> > Is such a configuration considered valid?
>
> Definitely no.
>
> > Should QEMU
> > complain loudly about it and refuse to start?
>
> Yes, but we need a clean way to do it. Eduardo is working
> on a series that would expose to libvirt this info,
> Eduardo can you please confirm?
The work has two parts: 1) actually preventing invalid
configurations; 2) providing a query interface to let libvirt
know which configurations are valid.
My focus is on (2) (adding a query-device-slots interface), but
we surely need to address bugs related to (1) first, so both the
device-plugging code and slot-querying code are correct and agree
with each other.
--
Eduardo